diff --git a/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/PidType.java b/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/PidType.java
index 5a297be..0a22e02 100644
--- a/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/PidType.java
+++ b/src/main/java/eu/dnetlib/dhp/schema/oaf/utils/PidType.java
@@ -5,8 +5,58 @@ import org.apache.commons.lang3.EnumUtils;
 
 public enum PidType {
 
-	// Result
-	doi, pmid, pmc, handle, arXiv, nct, pdb,
+	/**
+	 * The DOI syntax shall be made up of a DOI prefix and a DOI suffix separated by a forward slash.
+	 *
+	 * There is no defined limit on the length of the DOI name, or of the DOI prefix or DOI suffix.
+	 *
+	 * The DOI name is case-insensitive and can incorporate any printable characters from the legal graphic characters
+	 * of Unicode. Further constraints on character use (e.g. use of language-specific alphanumeric characters) can be
+	 * defined for an application by the ISO 26324 Registration Authority.
+	 *
+	 *
+	 * DOI prefix: The DOI prefix shall be composed of a directory indicator followed by a registrant code.
+	 * These two components shall be separated by a full stop (period). The directory indicator shall be "10" and
+	 * distinguishes the entire set of character strings (prefix and suffix) as digital object identifiers within the
+	 * resolution system.
+	 *
+	 * Registrant code: The second element of the DOI prefix shall be the registrant code. The registrant code is a
+	 * unique string assigned to a registrant.
+	 *
+	 * DOI suffix: The DOI suffix shall consist of a character string of any length chosen by the registrant.
+	 * Each suffix shall be unique to the prefix element that precedes it. The unique suffix can be a sequential number,
+	 * or it might incorporate an identifier generated from or based on another system used by the registrant
+	 * (e.g. ISAN, ISBN, ISRC, ISSN, ISTC, ISNI; in such cases, a preferred construction for such a suffix can be
+	 * specified, as in Example 1).
+	 *
+	 * Source: https://www.doi.org/doi_handbook/2_Numbering.html#2.2
+	 */
+	doi,
+
+	/**
+	 * PubMed Unique Identifier (PMID)
+	 *
+	 * This field is a 1-to-8 digit accession number with no leading zeros. It is present on all records and is the
+	 * accession number for managing and disseminating records. PMIDs are not reused after records are deleted.
+	 *
+	 * Beginning in February 2012 PMIDs include extensions following a decimal point to account for article versions
+	 * (e.g., 21804956.2). All citations are considered version 1 until replaced.  The extended PMID is not displayed
+	 * on the MEDLINE format.
+	 *
+	 * View the citation in abstract format in PubMed to access additional versions when available (see the article in
+	 * the Jan-Feb 2012 NLM Technical Bulletin).
+	 *
+	 * Source: https://www.nlm.nih.gov/bsd/mms/medlineelements.html#pmid
+	 */
+	pmid,
+
+	/**
+	 * This field contains the unique identifier for the cited article in PubMed Central. The identifier begins with the
+	 * prefix PMC.
+	 *
+	 * Source: https://www.nlm.nih.gov/bsd/mms/medlineelements.html#pmc
+	 */
+	pmc, handle, arXiv, nct, pdb,
 
 	// Organization
 	openorgs, corda, corda_h2020, GRID, mag_id, urn,